Molecular pump and method therefor

ABSTRACT

Method for constructing molecular pumps and high-vacuum molecular pumps improved according to that method, comprising a rotating drum with slight play within a stator whose inside face is cylindrical. The improvement consists in increasing the number of threads formed on the inside face of the stator or the rotor or on both adjacent faces, going from the suction to the discharge.

United States Patent 11 1 1111 3,912,415 Maurice Oct. 14 1975 MOLECULAR PUMP AND METHOD 3,697,190 10 1972 Haentjens 415 72 THEREFOR 3,802,794 4/1974 Gehring et al. 415/72 [75] Inventor: Louis Maurice, Paris, France [73] Assignee: Compagnie lndustrielle des Primary Exami' 1er C Husar Telecommunications Cit-Mead, Asszstant Exammer-L. J. Casaregola France Attorney, Agent, or Firm-Craig & Antonelli [22] Filed: Mar. 21, 1974 [21] Appl. NO.: 453,498 57 ABSTRACT 30 Foreign Application Priority Data Method for constructing molecular pumps and highvacuum molecular pumps improved according to that Mar. 21, 1973 France 73.10183 method, comprising a rotating drum with Slight p y [52] U S Cl 415 415/90 within a stator whose inside face is cylindrical. The [51] F6ld1/36 improvement consists in increasing the number of [58] Fieid "415/71 72 threads formed on the inside face of the stator or the 1 rotor or on both adjacent faces, going from the suction to the discharge. [56] References Cited UNITED STATES PATENTS 5/l924 Holweck 415/72 14 Claims, 3 Drawing Figures US. Patent Oct. 14,1975 Sheet10f2 3,912,415

FIG .1 PRIOR ART MOLECULAR PUMP AND METHOD THEREFOR BACKGROUND OF THE INVENTION The present invention concerns a method for manufacturing improved molecular high vacuum pumps having a cylindrical drum, of the multi-thread type, as well as pumps produced according to that method.

A molecular pump consisting of a cylindrical drum rotating at high speed with very slight clearance inside a stator having the same axis of symmetry as the drum (Holweck pump) has been known for nearly fifty years, now. A helix whose cross-section decreases from the part subjected to the highest vacuum towards the zone where the vacuum is slighter has been formed on the inside wall of the drum.

The pump thus produced industrially is in fact constituted by two half pumps facing each other and actually operating in parallel.

In the first pumps thus produced, the hollowed groove part of the inside face of the stator is a helix having a single thread.

This single-thread type of pump has been abandoned subsequently because of the slight pumping speed which it enabled, which reached only a few liters per second and also because of the low operating reliability, the extremely slight clearance between the rotor and the stator leading to frequent seizing therebetween.

Since about fifteen years, now, a great improvement has been produced by forming, on the stator or on the rotor of the pump or on the rotor and the stator, several helixes parallel to one another, thus giving a multithread pump.

But the hopes founded on that new structure have been frustrated because of the still insufficient performances and this improvement has not been put to industrial applications except for devices whose discharge is practically zero.

The tests carried out by the inventor have nevertheless proved that it is possible to produce multi-thread drum type pumps giving a satisfactory discharge in the order of 100 liters per second while remaining within reasonable bulk limits. Nevertheless, when such a pump is calculated to give a high discharge, its rate of compression is lowered to a prohibitive extent and it loses a great part of its advantage.

The tests carried out by the inventor have indeed proved that according to theory, there was, for a given active surface of a cylindrical drum type pump having optimum characteristics, an opposition between the characteristic of the discharge and that of the compression ratio; thus, it has been observed that a pump having an active surface defined by the diameter and the length of the cylindrical drum, a given rotor-to stator clearance and having a determined rotation speed, gives a satisfactory discharge only when the compression ratio becomes insufficient. Thus, for example, a multi-thread pump according to the art known up till now, rotating at 24,000 rpm and whose cylindrical rotor has a diameter of 160 mm and a length of 100 mm effectively discharges 120 liters per second, but then the compression ratio is for nitrogen and this is insufficient. On the other hand, if it is designed to give a compression ratio of 10 for nitrogen, the discharge is only 40 liters per second, this constituting an insufficient performance for usual applications of the vacuum technique.

The inventor has therefore sought to improve the compression ratio which may be obtained with a molecular vacuum pump having a cylindrical drum of the multi-thread type, capable of giving a satisfactory discharge, for a given active surface, rotation speed and clearance between the rotor and the stator.

It has been observed by the inventor that for a given clearance between the cylindrical rotor and the stator the impossibility of obtaining high compression ratios in a pump which is, moreover, designed for a high discharge, was greatly due to the increase in the leaks between two parallel threads which assume a preponderant value in the part of the pump in which the discharge occurs and in which the compression ratio per unit length of drum reaches a high value.

To reduce these leaks, various measures may be considered, such as, for example, that which consists in reducing the clearance between the rotor and the stator, but these measures, besides the fact that they lead in general to the considerable increasing of the cost of machining, reduce the operational reliability of the pump to a prohibitive extent, the least foreign body, the least expansion of one of its parts or the least mechanical deformation causing seizing, as occurred fifty years ago in the first Holweck pumps.

SUMMARY OF THE INVENTION The method and the device developed by the inventor enable, in a cylindrical drum type pump designed for a satisfactory discharge, an increase of the compression ratio without reducing the operational clearance thereof and without appreciably increasing the cost of machining.

The object of the invention is a method enabling improvement of the operation of high vacuum molecular pumps having a cylindrical drum, of the multi-thread type, characterized in that the number of threads is increased from the suction part towards the discharge part thereby sub-dividing each of the grooves comprised between two successive threads into several separated channels.

The device resulting from the implementing of the above method is therefore a high vacuum molecular pump comprising a cylindrical drum rotating at high speed with slight clearance in a stator whose inside face is cylindrical, comprising, moreover, several helical grooves formed on at least one of two adjacent faces which are parallel to each other, separated by low walls beginning at the inlet of the pump where suction occurs and ending at one of the ends of the drum where discharge occurs, characterized in that over one part of its length, each groove is subdivided into several narrower channels by low walls which are parallel to each other.

It may be an advantage to subdivide each gaseous flow passing along helical grooves first into two channels by means of a low central wall, them, further downstream, to subdivide again once or twice each of the channels by means of low walls beginning nearer to the outlet.

Simplifications in machining may lead to a reducing of the subdividing of each helical groove to only once, into two or three channels by means of one or two intermediate low walls respectively which are not very long.

Moreover, central suction pumps have been constructed comprising, on either side of a central suction volume, multiple grooves with reverse pitches on each side of the central suction, each of them being subdivided in the vicinity of the end of the drum into several channels. Such a device operates as a device formed by two half-pumps operating in parallel and enables the discharge to be increased.

In the majority of the cases, it has nevertheless been possible to produce a molecular pump having a satisfactory discharge with only one suction means and only one discharge means, placing the suction means at the level of one of the ends of the drum, discharge occurring at the other end.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The improvement produced by means of the invention will be more easily understood on referring to the detailed description of two embodiments in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a multi-thread cylindrical drum device substantially according to the known art;

FIG. 2 shows the developed view of one of the grooves of the stator according to one embodiment of the invention;

FIG. 3 shows a developed view of one of the grooves of the stator according to another embodiment of the invention.

On referring to FIG. 1 showing diagrammatically the essential parts of a cylindrical drum molecular pump, it will be observed that a simple multi-thread pump comprises a stator l and a cylindrical rotor 2 whose axis of rotation 3 merges with the geometrical axis of the stator. According to FIG. 1, the upper part of the drawing shows the suction side of the pump, the discharged gaseous flux leaving through the bottom part of the drawing. In that pump, the threads have been traced on the cylindrical stator. The inlets of the multiple grooves 4, 5, 6, 7, 8, 9, l0, and 11 appear in the thickness of the stator l.

A low wall appears between the grooves. Thus, between the grooves 8 and 9, there is a low wall 12; there is a low separation wall 13 between the grooves 9 and 10. The low walls, such as 12 and 13 form, on the cylindrical inside face of the stator, helixes limiting, in the stator, eight parallel helical grooves. The depth of the grooves decreases from the inlet to the discharge.

Such a pump having eight parallel grooves traced on the stator giving a discharge of 120 liters per second, comprising a cylindrical drum whose diameter is 160 mm and whose height is 100 mm with a clearance in the order of 0.04 mm and rotating at a speed of 24,000 rpm gives a compression ratio of only as has been stated above, because of the very great leakages which are produced therein in the part where the compression ratio is at its maximum.

If the number of threads in the zone where the compression ratio is high is increased by subdividing each of the helical grooves by means of intermediate low walls, the losses are reduced and a very substantial improvement of the compression ratio is obtained. A molecular pump according to the invention in which the number of threads increases from the inlet to the discharge such as that described with reference to FIG. 2

or that which is shown in FIG. 3 makes it possible to ob- 6 tain a compression ratio in the order of 10 without reducing the discharge.

FIG. 2 shows a developed illustration of the trace formed on the stator of the pump. The left-hand low wall in FIG. 2 being, for example, the low wall 12 in FIG. 1 and the right-hand low wall being the low wall 13, the gap between the two low walls being constituted by the groove 9.

According to the invention, during the machining of the groove 9, a low central wall 15 dividing the helical groove 9 into two channels 16 and 17 is provided in the region where the compression ratio rises substantially, starting from the first third of the length of the thread for example, a low central wall 12 dividing the helical groove 9 into two channels 16 and 17. The reduction of the leakages thus obtained is already spectacular and the compression ratio of the pump is improved in great proportions. It may, however, be an even greater advantage to again subdivide the channels 16 and 17 by means of two low walls 18 and 19 which are shorter to make four channels 20, 21, 22, and 23 appear at the outlet, whereas at the inlet, there was only one helical groove 9.

FIG. 3 shows another embodiment also according to the invention in which the helical groove comprised between the low walls 12 and 13 has been subdivided substantially at the third of the length of the thread into three channels 24, 25 and 26 by means of two intermediate low walls 27 and 28 having the same length.

In certain embodiments, low walls such as 27 and 28 have been arranged between low walls such as 12' and 13 exclusively in the vicinity of the discharge, thus limiting channels such as 24, 25 and 26, which are short.

Although the devices which have just been described appear to afford the greatest advantages for the implementing of the invention in a particular technical situation, it will be understood that various modifications may be made thereto without going beyond the scope of the invention, more particularly by modifying the depth of the channels to the level of the complementary low walls according to the invention.

What I claim is:

l. A method for improving the operation of a high vacuum molecular pump of the multi-thread type comprising increasing the number of threads at a discharge portion of a multi-thread high vacuum molecular pump by forming at least one low wall between two successive threads, said at least one low wall being parallel to said two threads and being formed with a length shorter than the length of the groove between said two threads, thereby subdividing respective grooves formed between two successive threads into at least two additional channels at the discharge portion.

2. A method according to claim 1, wherein said molecular pump is constructed substantially cylindrical with said discharge portion being at one end of the cylindrical construction.

3. A method according to claim 1, wherein said molecular pump is constructed substantially cylindrical with a central suction portion and said discharge portion being provided at each end of the cylindrical construction, said molecular pump having multi-thread portions of reverse pitch on each side of said central suction portion.

4. A high vacuum molecular pump comprising:

a stator,

a rotor provided for rotation with slight clearance with respect to said stator,

a plurality of helical grooves provided at a surface of at least one of the stator and rotor, said surface facing the other of the stator and rotor, said plurality of helical grooves being formed by a plurality of parallel walls extending from a suction inlet means to a discharge outlet means, and

at least one additional wall provided between successive ones of said plurality of parallel walls for subdividing each of said plurality of helical grooves in the vicinity of said discharge outlet means, said at least one additional wall having a length shorter than the length of each of said plurality of helical grooves.

5. A molecular pump according to claim 4, wherein said at least one additional wall extends parallel to said plurality of parallel walls over a portion of the lengths thereof from said discharge outlet means.

6. A molecular pump according to claim 4, wherein said plurality of helical grooves are subdivided exclusively on a stator surface.

7. A molecular pump according to claim 4, wherein said plurality of helical grooves are subdivided exclusively on a rotor surface.

8. A molecular pump according to claim 4, wherein one additional wall is provided between successive ones of said plurality of parallel walls to subdivide each of said plurality of helical grooves into two channels, said one additional wall having a rectangular crosssection.

9. A molecular pump according to claim 8, wherein two further additional walls are provided to subdivide each of said two channels into two further channels, said two further additional walls each being shorter in length than said one additional wall.

10. A molecular pump according to claim 4, wherein two additional walls are provided between successive ones of said plurality of parallel walls to subdivide each of said plurality of helical grooves into three channels, said two additional walls each having a rectangular cross-section.

11. A molecular pump according to claim 10, wherein said two additional walls are of the same length.

12. A molecular pump according to claim 4, wherein said stator is constructed cylindrically, and said rotor is constructed as a cylindrical drum for concentric rotation within said stator.

13. A molecular pump according to claim 12,- 

1. A method for improving the operation of a high vacuum molecular pump of the multi-thread type comprising increasing the number of threads at a discharge portion of a multi-thread high vacuum molecular pump by forming at least one low wall between two successive threads, said at least one low wall being parallel to said two threads and being formed with a length shorter than the length of the groove between said two threads, thereby subdividing respective grooves formed between two successive threads into at least two additional channels at the discharge portion.
 2. A method according to claim 1, wherein said molecular pump is constructed substantially cylindrical with said discharge portion being at one end of the cylindrical construction.
 3. A method according to claim 1, wherein said molecular pump is constructed substantially cylindrical with a central suction portion and said discharge portion being provided at each end of the cylindrical construction, said molecular pump having multi-thread portions of reverse pitch on each side of said central suction portion.
 4. A high vacuum molecular pump comprising: a stator, a rotor provided for rotation with slight clearance with respect to said stator, a plurality of helical grooves provided at a surface of at least one of the stator and rotor, said surface facing the other of the stator and rotor, said plurality of helical grooves being formed by a plurality of parallel walls extending from a suction inlet means to a discharge outlet means, and at least one additional wall provided between successive ones of said plurality of parallel walls for subdividing each of said plurality of helical grooves in the vicinity of said discharge outlet means, said at least one additional wall having a length shorter than the length of each of said plurality of helical grooves.
 5. A molecular pump according to claim 4, wherein said at least one additional wall extends parallel to said plurality of parallel walls over a portion of the lengths thereof from said discharge outlet means.
 6. A molecular pump according to claim 4, wherein said plurality of helical grooves are subdivided exclusively on a stator surface.
 7. A molecular pump according to claim 4, wherein said plurality of helical grooves are subdivided exclusively on a rotor surface.
 8. A molecular pump according to claim 4, wherein one additional wall is provided between successive ones of said plurality of parallel walls to subdivide each of said plurality of helical grooves into two channels, said one additional wall having a rectangular cross-section.
 9. A molecular pump according to claim 8, wherein two further additional walls are provided to subdivide each of said two channels into two further channels, said two further additional walls each being shorter in length than said one additional wall.
 10. A molecular pump according to claim 4, wherein two additional walls are provided between successive ones of said plurality of parallel walls to subdivide each of said plurality of helical grooves into three channels, said two additional walls each having a rectangular cross-section.
 11. A molecular pump according to claim 10, wherein said two additional Walls are of the same length.
 12. A molecular pump according to claim 4, wherein said stator is constructed cylindrically, and said rotor is constructed as a cylindrical drum for concentric rotation within said stator.
 13. A molecular pump according to claim 12, wherein said suction inlet means and discharge outlet means are provided at opposite ends of the cylindrically constructed structure, and said plurality of helical grooves are subdivided at said discharge outlet means.
 14. A molecular pump according to claim 12, wherein said suction inlet means is provided at a central portion of the cylindrical structure and said discharge outlet means is provided at each end of said cylindrical structure, said plurality of helical grooves extending with reverse pitch on each side of said central portion to said discharge outlet means. 